In portable or handheld mobile communications terminal devices such as personal digital cellular (PDC) or personal handyphone systems (PHS) or the like, radiocommunications are performed by use of carrier waves in microwave bands at frequencies of 1 gigahertz (GHz) or higher. Due to this, power amplifier circuitry for transmit signals and pre-amplifier circuitry for receive signals are typically designed to employ gallium arsenide (GaAs) MESFETs that operate at higher speeds than standard silicon transistors.
General teachings about mobile communications terminal devices are found in several printed publications including, for example, “NIKKEI ELECTRONICS,” by Nikkei BP Corp., Apr. 16, 1990 (No. 497), p. 121.
While this mobile communications terminal device requires relatively large electrical power of approximately 1 watt (W) for over-the-air signal transmission, it is also required that the device be smaller in size and longer in operation time period on a battery in order to increase the portability of such mobile communications terminal. In view of this, use of battery-based single power supply drive scheme is preferable, which in turn requires low power consumption in a viewpoint of guarantee of long term operabilities.
Incidentally, in cases where GaAs MESFETS are utilized at high frequency bands, n-channel type MESFETS are generally employed in order to take full advantage of inherent significance of electron mobility therein. Accordingly, the following description will be devoted to the case of n-channel MESFETS, except as otherwise stated to the contrary.
In addition, in prior art MESFETs, those of the depression type that are relatively deep in threshold voltage (e.g. Vth=−1V, or more or less) are used in order to gain a significant amplification degree.
In case the MESFET of relatively deep Vth is used with its source grounded, it should be required that a gate bias of negative potential be applied thereto, which in turn requires separate use of a negative power supply voltage in addition to a positive power supply voltage. The amplifier system requiring such power supplies of both the positive and negative polarities is incapable of being driven by a single power supply—when an attempt is made to forcibly drive the system by using a single or unitary power supply, a specific scheme will be required for employing a DC-DC converter to generate from a positive power supply a negative voltage for use as the negative power supply.
Unfortunately the DC-DC converter employment scheme does not come without accompanying penalties as to an increase in power consumption and also an increase in parts mount area, which will become contradictory to the need for small size and long term battery drivability posed on mobile communications terminals.
Then, a need arises to consider employment of special circuitry for applying a gate bias voltage of zero volts or of positive polarity, which circuitry is typically designed to make use of certain GaAs MESFETs of either relatively shallow depression type or enhancement type with Vth being positive in polarity, as amplifying elements for use in a power amplifier circuit of mobile communications terminals.
In view of the fact that a GaAs MESFET constitutes a Schottky junction FET—in other words, a gate and source make up a Schottky diode—while an n-channel MESFET is used with its source grounded, application of a positive voltage to the gate would result in creation of a forward voltage with respect to the Schottky diode. This in turn makes it necessary that a positive voltage capable of application to the gate must be less than or equal to a specified voltage (Vf) at which a gate current (forward current) behaves to rapidly increase. This requirement comes because even upon applying a gate voltage of Vf or higher, a depletion layer underlying a gate electrode has already disappeared leading to an inability to control a drain current which can result in saturation of the drain current. On the other hand, a minimal value of the gate voltage capable of being applied in the negative direction becomes near or around the Vth value. This can occur because even when applying a gate voltage of less than or equal to Vth, a channel region has already been cut off by a depletion layer so that any drain current is no longer flowing therein.
In brief, while a linear region which permits the drain current to vary with a change in gate voltage is needed in order to take out the drain current of a MESFET as the intended amplified signal, the use of this region means that the gate voltage must fall within a limited range of from Vth to Vf.
Accordingly, when compared to a deep Vth depression type MESFET, MESFETs of the shallow Vth depression type or positive Vth enhancement type become narrower in range insuring gate voltage applicability. Generally the drain current gets larger when applying the gate voltage maximally; thus, the drain current tends to increase in amplitude in a way proportional to the amplitude of such gate voltage. Due to this, in the case of the MESFETs of relatively shallow depression type or enhancement type, it will possibly happen that any sufficient drain current is hardly obtainable. This would result in that the intended output or gain of the amplifier system is by no means attainable during a high frequency operation thereof, which leads to occurrence of a serious bar to the quest for higher performance in mobile communications terminals.
On the other hand, as has been recited in Japanese printed matter such as for example “COMPOUND SEMICONDUCTORS,” Nikkan Kougyou Shinbun-Sha (this means in English “Daily Engineering Newspaper Corp.”), Jan. 30, 1986 at p. 164, the current density, J, of a forward current flowing between a metal and a semiconductor that are in Schottky junction is given as:J=A*T2exp(−qφB/kT)(exp(qV/nkT)−1),where “A*” is the effective Richardson constant, T is the absolute temperature (K), q is the elementary charge carrier, φB is the Schottky barrier (V), k is the Boltzmann's constant, V is the applied voltage (V), and n is the ideal parameter or coefficient, which is expected to fall within a range of 1.0 to 1.3 when the Schottky junction is superior.
Assuming that exp (qV/nkT) is established and “n” is nearly equal to 1, the current density J behaves to increase exponentially at or near a point whereat V goes beyond φB as readily appreciated by those skilled in the art to which the invention pertains. Such situation is equivalent to the phenomenon that a gate current rapidly increases with an increase in gate voltage in source-grounded MESFETs. In short, Vf is strongly related to φB—the greater φB, the larger Vf. Accordingly, it may be considered that the use of those materials with large values of φB, for the gate electrode is effective in order to increase the Vf value to thereby likewise increase the range of application of the gate voltage exhibiting amplification functionality.
Regrettably it has been known among experts in the art that even when a metal with Schottky junctionability is formed on the surface of GaAs, φB does not vary in accordance with the kind of a metal, that is, the work function of such metal, and thus φB remains almost constant. It is considered that this is owing to greatness of the surface energy level density on GaAs surfaces or alternatively pinning effects occurring due to creation of an intermediate layer.
In prior art n-channel GaAs MESFETs employed in many cases, the gate electrode is typically made of a tungsten silicide (WSi)-based material, wherein even if this gate electrode is modified to replace it with either aluminum (Al) or molybdenum (Mo) by way of example, the resultant Schottky barrier φB will never change significantly due to the presence of pinning effects. For this reason, it remains difficult to increase the gate voltage applicable range by increasing Vf, which in turn makes it difficult for those MESFETs of the relatively shallow depression type or enhancement type to provide a sufficient drain current for improvement in output or gain of the amplifier system during a high frequency operation thereof to thereby increase the performance of mobile communications terminals.
Additionally in cases where n-channel GaAs MESFETs of the relatively shallow depression type or enhancement type are employed with the source grounded, the resulting range of a gate voltage becomes narrower, which causes the stability of such gate bias voltage to significantly affect the signal-to-noise ratio (SNR) concerned. In view of this, the stability of power supply voltage will especially become important.
It is therefore an object of the present invention to improve the gain of a high frequency power amplifier system that is inherently designed to be driven by a single power supply.
It is another object of this invention to reduce electrical power as consumed by the high frequency power amplifier system.
It is still another object of the invention to improve the stability of such high frequency power amplifier system.
It is a further object of the invention to increase an output of a mobile communications terminal device offering drivability on a single power supply with low power consumption—namely capable of exhibiting long term operation on a battery—while improving performance.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.